1. Field of the Invention
The present invention relates to a lighting circuit for supplying an AC voltage to a discharge lamp by converting a DC voltage from a DC power source circuit. The circuit has a current detection means to detect a current flowing to the discharge lamp in the DC power source circuit.
2. Related Art
A lighting circuit for a discharge lamp, having a DC power source circuit, a DC-AC conversion circuit, and a starter circuit, is known.
For example, a lighting circuit has a DC-DC converter in the DC power source circuit, and a DC-AC converter containing a driver circuit and a full bridge circuit having two pairs of semiconductor switch elements for conducting switching control. The output voltage of the DC-DC converter is converted into a rectangular wave-shaped voltage by the full bridge circuit and supplied to the discharge lamp.
To detect a current flowing to the discharge lamp, a shunt resistance is inserted between the discharge lamp and the ground, and a signal after voltage conversion is detected.
However, the above configuration is subject to a surge voltage and may face safety problems.
When a discharge lamp is lighted, a surge voltage may be generated by a distributed or stray capacitor. For a mobile lighting circuit, a high voltage pulse of several kV""s to tens of kV""s generated by a starter circuit is sent to the discharge lamp through a high voltage cord. A shield is wrapped around the cord to suppress electromagnetic interferences brought on by the generation of radio frequency noise. The stray or distributed capacity are also charged between the high voltage cord and the car body or the shield, both at ground. Thus, when the discharge lamp breaks down and conducts after receiving the high voltage pulse, the electrical charge accumulated in the stray capacity or the distributed capacity discharges through the shunt resistance. Consequently, the shunt resistance or elements of the current detection circuit provided downstream to the resistance can be damaged.
Further, if the wiring between the shunt resistance and the discharge lamp short circuits by contacting, for example, the car body, the current stops flowing through the shunt resistance and flows out through the car body (ground potential). Since no current is flowing to the discharge lamp, the control circuit ramps the power supply to provide more current to the discharge lamp. A fail-safe circuit then detects the above condition to stop the operation. Circuits such as a fail-safe circuit increase costs, size, and the number of parts of the lighting circuit.
According to the present invention, a discharge lamp lighting circuit comprises a DC power source circuit to output a DC voltage, a DC-AC conversion circuit for converting the DC voltage into an AC voltage to be supplied to a discharge lamp, a current detection circuit to detect a current flowing to the discharge lamp, and a control circuit to control the output voltage of the DC power source circuit based on a status detection signal of the discharge lamp including a detection signal from the current detection circuit. The present invention can have the following structures, (A) to (C).
(A) A positive polarity voltage and a negative polarity voltage respectively from two output terminals of the DC power source circuit are sent to the DC-AC conversion circuit. A plurality of switch elements provided in the DC-AC conversion circuit for switching these output voltages are alternately operated by their driving circuits to generate AC voltage, which is supplied to the discharge lamp.
(B) One electrode of the discharge lamp is connected to one of the output terminals of the DC-AC conversion circuit through an inductive load, and the other electrode is connected to the ground.
(C) A current detection element to detect a current flowing to the discharge lamp is provided in the DC power source circuit, and a signal from the current detection element is received by the current detection circuit.
According to the present invention, the current detection element for detecting a current flowing to the discharge lamp is provided in the DC power source circuit and not connected to the discharge lamp. For example, the element may be inserted between the output terminal and the ground. Thus, problems associated with the surge voltage generated when the discharge lamp is lighted can be averted. Also, a protection circuit for protecting circuits is not needed because the current can be detected by the current detection element even if the current leaks to the car body.